New Avenues in Atmospheric Modelling of Exoplanets
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In this thesis I explore various aspects of atmospheric characterisation of exoplanets with the primary goal of understanding their chemical compositions and physical processes. My research led to the development of new self-consistent models of exoplanetary atmospheres, a new paradigm for atmospheric retrievals of thermal emission spectra, as well as chemical detections using both high-resolution Doppler spectroscopy as well as low-resolution transit spectroscopy.
I firstly computed the molecular and atomic cross sections of various species prevalent in the atmospheres of such exoplanets in order to compute their spectra. The absorption cross sections were calculated through the broadening of spectral lines obtained from high resolution line lists. These cross sections and subsequent spectral models have led to the detections of numerous chemical species (HCN, TiO, Li, Na, K, CO, and H
Recent advances in observations have heralded the need for accurate models of exoplanetary atmospheres. I have built a new self-consistent atmospheric model, GENESIS, custom built for exoplanets and demonstrated for irradiated and non-irradiated atmospheres over a wide range of atmospheric parameter space. The model treats line-by-line radiative transfer through the Feautrier method and radiative-convective equilibrium through the Rybicki Complete Linearisation method in a plane parallel atmosphere. This model allows for a detailed exploration of radiative processes and chemical compositions and their effects on observed emission spectra. I compared this model against several others in the literature and found good agreement between the atmospheric properties and emission spectra.
Thermal inversions have been seen on the dayside atmospheres of some hot Jupiters and have been predicted to be caused by TiO or VO due to their visible opacity. I used the GENESIS model to investigate the effect of visible opacity and deduced that many new species (AlO, CaO, NaH and MgH), hitherto unexplored, are also capable of causing thermal inversions on hot Jupiters. I have explored the effect of these species as a function of their overall atmospheric abundance as well as determining the required abundance for each of these species to form an inversion. Secondly, I show that a low infrared opacity caused by a low H
I have also developed a new hybrid retrieval method for exoplanetary emission spectra, HyDRA. This uses the latest atmospheric modelling tools to fit the observed spectra of exoplanet atmospheres. We explore a wide range of parameter space and determine the temperature profile and abundances of various species present in the dayside atmosphere through the emission spectra. These retrieved abundances are then used to explore disequilibrium processes which may be present through integration into the GENESIS self-consistent model. Such a framework allows constraints on departures of the temperature structures from radiative-convective equilibrium as well as chemical compositions from thermochemical equilibrium. I explored HST and Spitzer observations of WASP-43b and confirmed the data were in agreement with radiative-convective equilibrium in the dayside atmosphere.
The HyDRA retrieval framework has also been extended to model the atmospheres of ultra-hot Jupiters with temperatures in excess of 2500~K. Such high temperatures can cause molecular species such as H
I have also used the HyDRA retrieval framework to perform a set of homogeneous retrievals for eight well known hot Jupiters with high precision HST WFC3 spectra. These planets all also have Spitzer observations which I also use to explore the atmospheric temperature profile and chemical composition, in particular explore the H
Finally, I have used the GENESIS model to enable chemical detections of molecular species using high resolution Doppler spectroscopy of hot Jupiters. I generated high resolution emission spectra of the hot Jupiters HD189733b and HD209458b for cross correlation with the data obtained with the VLT CRIRES spectrograph. This helped us find evidence for H